A magnetic flux generator (hereinafter called a “sensor magnet”) is installed in a rotatable body, and a magnetic sensor is installed in a location within the reach of magnetic flux generated by the magnetic flux generator. It is known that, when the rotatable body rotates, the angle of the magnetic flux generated by the sensor magnet also rotates, and that the rotational position (rotational angle) of the rotatable body can be measured by detecting the angle of the magnetic flux through the use of a magnetic sensor.
Here, the magnetic sensor is broadly divided into two groups: a magnetic-field-intensity-measurement sensor which outputs a signal corresponding to the intensity of magnetic field, and a magnetic-field-angle-measurement sensor which outputs a signal corresponding to the angle of magnetic field. Since the magnetic-field-angle-measurement sensor measures the angle of magnetic field as a vector, it is also called a vector-type magnetic sensor.
The magnetic-field-angle-measurement sensor includes one which employs a Hall-effect element as a magnetic-field detection element, one which employs a magneto-resistance element, and others.
The Hall-effect element itself is an element which outputs a signal corresponding to the magnetic field intensity. However, it is possible to output a signal corresponding to an angle of magnetic field, by measuring the spatial difference of the magnetic field intensity with the use of plural pieces of Hall-effect elements, and detecting a cosine component (COS component) and a sine component (SIN component) of the magnetic field.
There is also a sensor which measures a magnetic-field angle by employing a suitably-shaped magnetic material and plural pieces of Hall-effect elements. This type of magnetic sensor concentrates a magnetic flux with the magnetic material and converts the magnetic-field angle into a difference in the magnetic field intensity, which is then measured by the plural pieces of Hall-effect elements.
In this way, there exist various kinds of magnetic sensors of the type of a magnetic-field-angle-measurement sensor which is configured with Hall-effect elements and outputs a signal corresponding to the magnetic-field angle.
The magneto-resistance element is an element of which the electrical resistance changes depending on the intensity of magnetic field or the angle of magnetic field. The magneto-resistance element includes an anisotropic magneto-resistance element (hereinafter called an “AMR element”), a giant magneto-resistance element (hereinafter called a “GMR element”), and a tunneling magneto-resistance element (hereinafter called a “TMR element”).
An AMR element changes electrical resistance depending on the angle between the angle of magnetic field and the angle of electric current. By combining appropriately an element with a different angle of electric current, a signal corresponding to the angle of magnetic field is outputted. A GMR element has a configuration in which a pinned magnetic layer and a free magnetic layer are laminated with a spacer layer sandwiched therebetween. By combining appropriately an element with a different angle of spin (angle of magnetization) in the pinned magnetic layer, a signal corresponding to the angle of magnetic field is outputted. The GMR element which has a pinned magnetic layer is also called a spin-valve type GMR element.
One of the advantages of the rotational angle sensor which employs a magnetic sensor is that the rotational angle sensor is a non-contact type. The non-contact type refers to that the rotatable body and the sensor as a detector for detecting a rotational position are mechanically non-contact. That is, because of being not mechanically in contact, even when the rotatable body rotates at a high speed and is employed over a long period of time, mechanical wear does not occur; accordingly a reliable sensor is realized.
Another advantage of the rotational angle sensor which employs the magnetic sensor is that it is possible to increase the distance between the rotatable body and the sensor. This is originated from the fact that the effect of magnetic field reaches up to a comparatively long distance. For example, when a magnetic sensor employing a GMR element and a sensor magnet (magnetic flux generator) of a neodymium magnet are combined, the distance between them can be increased up to about 5-15 mm. As compared with this, in a resolver which measures a rotational angle by change of reluctance, the distance between a rotatable body and a detector (sensor) is as narrow as about several 100 μm. The fact that the large distance between a rotatable body and a sensor is allowed has advantages such as improving design flexibility or relaxing manufacturing tolerance of the rotation machine (for example, a motor) which uses a rotatable body as a component.
Another advantage of the rotational angle sensor which employs a magnetic sensor is that a non-magnetic body may exist between a rotatable body and a sensor. Since magnetic susceptibility χ of a non-magnetic material is almost zero (|χ|<0.1), relative permeability μr is approximately 1 and is nearly equal to the relative permeability of the air. Therefore, even if there exists a non-magnetic material, the angle of magnetic field changes only to a negligible degree. This fact leads to an advantage for improving the design flexibility of a rotation machine (for example, a motor) which uses a rotatable body as a component.
In a rotational angle sensor in the past, in cases where a non-magnetic conductor (electrical conductor) is arranged between a magnetic flux generator installed in a rotatable body and a magnetic sensor, when the rotatable body rotates or moves at high speed, there occurs the problem that it is difficult to measure an angle of magnetic field correctly due to an eddy current generated. That is, when a magnetic flux generator rotates at high speed, the magnetic field in the position of the conductor changes in time and an eddy current is generated inside the conductor. The generated eddy current induces an eddy-current-based magnetic field and the magnetic-field distribution becomes different from the original magnetic-field distribution which is generated by the magnetic flux generator. Therefore, it becomes difficult for the magnetic sensor to detect correctly the angle of magnetic field which the magnetic flux generator itself generates.
On this problem, in cases where a magnetic flux generator (sensor magnet) rotates and produces the influence of an eddy current, Patent Literature 1 prevents the generation of an eddy current by employing a non-conductive material (electrically non-conductive material), such as a ceramics, as a non-magnetic material arranged between the rotatable body and the magnetic sensor.